Donald Trump wants to put the screws to H-1B visas, which are given to foreign professionals who perform specialty occupations in the U.S.

During the Republican debate on Thursday, the Republican front-runner admitted that he has “used” H-1B visas as a businessman because he could (a recent investigation shows that he personally benefited from this program). “I know the H-1B very well,” he said. “And it’s something that I frankly use and I shouldn’t be allowed to use it. We shouldn’t have it. Very, very bad for workers.” Trump went on to argue that he would prevent other executives from leveraging this option if elected president.

Trump’s immigration plan, released last fall, argues that employers who sponsor these H-1B visas should be required to pay workers much more money than they currently do. He argues that it would dissuade employers from hiring foreign workers, if it were even more expensive.

Those views have freaked out Silicon Valley’s wealthiest executives, to say the least.

Trump specifically called out Facebook CEO Mark Zuckerberg in the plan. Zuckerberg and other tech CEOs have called for the expansion of the H-1B visa program to attract more skilled engineers from abroad. Zuckerberg even started a lobbying group called Fwd.us to push for immigration reform.

“Attracting entrepreneurs and human capital is critical to the tech community,” said Todd Schulte, president of fwd.us during a conference call in September of 2015. Schulte has pushed for a startup visa program, similar to the one offered in Canada that specifically targets foreign entrepreneurs.

Earlier this week, tech CEOs joined up with leaders of the Republican establishment on a private island for a secretive rendezvous about how to derail Trump’s campaign. It’s not clear whether immigration was discussed at that meeting.